What does Genesis 4:7 mean?
In the previous verse, God asked Cain why he felt so much anger when God rejected his offering. God had shown favor on his brother Abel and his offering, which seems to have enraged Adam and Eve's firstborn son. God now speaks a wise warning to Cain. Cain will not heed it, but it is a warning for us as well.God asks Cain to adjust his understanding of what is good to God's understanding of goodness. If Cain does well by God's standard, God will accept him. In other words, there is no reason for Cain to be angry about God's rejection. The cure for that rejection is obedience: if Cain does what is right, God will accept him.
If Cain insists on setting his own standards for what is acceptable, sin "is crouching at the door." That poetic phrase captures the nature of our rebellion against God. Sin desires to own us, and our refusal to let God set the standard for right and wrong in our lives is the fast track to sin.
God acknowledges the reality of human nature. We are locked in a battle with sin's desire for us (or our desire to sin). God tells Cain he is responsible to win that battle, to rule over his sin. The Hebrew terms used in this verse are exactly the same ones spoken to Eve in Genesis 3:16. These are from the root words tashuwqah, translated "desire," and mashal, translated "rule over." Despite sin's "desire" for control over him, Cain must "rule over" his temptations and not give in.